Microbial reductive dechlorination is a naturally occurring transformation process tetrachloroethene (PCE) undergoes in aquifers. It contributes significantly to natural attenuation of chlorinated solvents. Stimulation of this process has been considered as a method of enhanced bioremediation. Experiments on the stimulation of reductive dechlorination were carried out in a large-scale quasi-2D sandbox model. The transformation of PCE to cis-1,2-dichloroethene (DCE) was attempted by inoculation with Dehalospirillum multivorans and that of DCE to ethene with a mixed culture. Ethanol used as the electron donor was introduced into the inlet of the domain, whereas water loaded with PCE was injected into a well. Limitations due to insufficient mixing could not be observed as high-permeability lenses enhanced the transverse exchange of the compounds. Both reductive dechlorination and competitive microbial reactions led to the acidification of the domain. The artificial aquifer was buffered by the concurrent injection of sodium sulfide with the electron donor. Under these conditions bioaugmentation of Dehalospirillum multivorans was successful, whereas stable dechlorination of DCE could not be achieved.
[1] Thermal response tests (TRTs) are a common field method in shallow geothermics to estimate thermal properties of the ground. During the test, a constantly heated fluid is circulated in closed tubes within a vertical borehole heat exchanger (BHE). The observed temperature development of the fluid is characteristic for the thermal properties of the ground and the BHE. We show that, when the BHE is installed in an aquifer with significant horizontal groundwater flow, this test can also be used for hydrogeological characterization of the penetrated subsurface. An evaluation method based on the moving line source equation and considering the natural occurring variability of the thermal transport parameters is presented. It is validated by application to a well-controlled, large-scale tank experiment with 9 m length, 6 m width, and 4.5 m depth, and by data interpretation from a field-scale test. The tank experiment imitates an advection-influenced TRT in a well-known layered aquifer. The field experiment was recorded with a 100 m deep BHE, installed in a gravel aquifer in southwest Germany. The evaluations of both experiments result in similar hydraulic conductivity ranges as determined by standard hydraulic investigation methods such as pumping tests and sieve analyses. Thus, advection-influenced TRTs could also potentially be used to determine integral hydraulic conductivity of the subsurface.
